Mechanical power density, spontaneous breathing indexes, and prolonged weaning failure: a prospective cohort study

A prospective observational study comparing mechanical power density (MP normalized to dynamic compliance) with traditional spontaneous breathing indexes (e.g., predicted body weight normalized tidal volume [VT/PBW], rapid shallow breathing index [RSBI], or the integrative weaning index [IWI]) for predicting prolonged weaning failure in 140 tracheotomized patients. We assessed the diagnostic accuracy of these indexes at the start and end of the weaning procedure using ROC curve analysis, expressed as the area under the receiver operating characteristic curve (AUROC). Weaning failure occurred in 41 out of 140 patients (29%), demonstrating significantly higher MP density (6156 cmH2O2/min [4402–7910] vs. 3004 cmH2O2/min [2153–3917], P < 0.01), lower spontaneous VT/PBW (5.8 mL*kg−1 [4.8–6.8] vs. 6.6 mL*kg−1 [5.7–7.9], P < 0.01) higher RSBI (68 min−1*L−1 [44–91] vs. 55 min−1*L−1 [41–76], P < 0.01) and lower IWI (41 L2/cmH2O*%*min*10−3 [25–72] vs. 71 L2/cmH2O*%*min*10-3 [50–106], P < 0.01) and at the end of weaning. MP density was more accurate at predicting weaning failures (AUROC 0.91 [95%CI 0.84–0.95]) than VT/PBW (0.67 [0.58–0.74]), RSBI (0.62 [0.53–0.70]), or IWI (0.73 [0.65–0.80]), and may help clinicians in identifying patients at high risk for long-term ventilator dependency.


Definitions of ventilatory indexes -Mechanical ventilation Ventilatory ratio (VR)
Ventilatory ratio (VR) is a surrogate of pulmonary dead space fraction and a simple bedside index of impaired efficiency of ventilation [1][2] : VEmeasured is the measured minute ventilation (mL/min), PaCO2-measured is the measured arterial pressure of carbon dioxide (mmHg), VEpredicted is the predicted minute ventilation calculated as predicted bodyweight x 100 (mL/min), and PaCO2ideal is the expected arterial pressure of carbon dioxide in normal lungs if ventilated with the predicted minute ventilation.PaCO2-ideal is set at 37.5 mmHg (5 kPa) for all patients.VR is a unitless ratio, with a value approximating one representing normal ventilating lungs.

Mechanical power (MP)
Mechanical power (MP) 3 provided by the ventilator in the pressure-controlled mode was calculated using a simplified formula, including respiratory rate (RR), tidal volume (VT), and Pmax With each breath the ventilator delivers, a certain amount of energy (Joule) is transferred to the patient's respiratory system.This energy is mainly used to overcome the airways' resistance, inflate the lungs, and expand the thoracic cage.

Mechanical power normalized to dynamic lung-thorax compliance (LTCdyn-MP)
MP normalized to dynamic lung-thorax compliance (LTCdyn-MP) was calculated using MP and dynamic lung-thorax compliance (LTCdyn) 6 : This formula accounts for different effects of changes in respiratory rate, inflation pressure (Pmax), and PEEP (and thus changes in ∆Paw) on delivered energy.Increasing RR leads to a linear rise in energy transfer while increasing pressure (concomitantly increasing tidal volume) results in an exponential increment in power 3 and stress intensity.

Power indexes of the respiratory system (PIrs)
LTCdyn-MP normalized to PaCO2 was calculated using LTCdyn-MP and PaCO2 similarly to the corrected minute ventilation [6][7] : Since PaCO2 is inversely proportional to minute ventilation (neglecting dead space fraction), exponent X approximates values between 1 and 2, depending on whether adjustments of ventilator settings are made for RR and/or Pmax/∆Paw to reach PaCO2-target, which was arbitrarily set at 45.0 mmHg (6.0 kPa, corresponding to the hypercapnic threshold) for all patients.Power indexrs estimate the LTCdyn-MP necessary for adequate alveolar ventilation (keeping PaCO2 below the hypercapnic threshold) 7 .

Rapid shallow breathing index (RSBI)
The rapid shallow breathing index, also known as the frequency-to-tidal volume ratio, is an index developed to test a mechanically ventilated patient's capacity to tolerate a trial of unassisted breathing 9 (referred to as readiness testing): RSBI (min -1 * L -1 ) = RR / VT Individuals who cannot tolerate such a trial usually develop a breathing pattern characterized by a high respiratory rate and low tidal volume a few minutes after disconnecting from the ventilator, resulting in an increased RSBI.Weaning trial failure is predicted by a score > 105 in such patients 9 .

Integrative weaning index modified (IWI modified)
The Integrative Weaning Index was developed to test a patient's capacity to tolerate a weaning trial and subsequent extubation 10 .We modified this index using dynamic instead of static respiratory system compliance for its computation:

Legend
The accuracy of each ventilatory variable in predicting weaning failure is presented as the area under the ROC curve with 95% confidence intervals.The redefined weaning failure criterion (PaCO2 > 50 mmHg) was met by 24 (17%) out of 140 patients.
Abbreviations: SBT, spontaneous breathing trial; P/F ratio, the ratio of partial pressure of oxygen to fraction of inspired oxygen; LTCdyn, dynamic lung-thorax compliance; LTCdyn-MP, mechanical power normalized to dynamic lung-thorax compliance; VT/PBW, tidal volume normalized to the predicted body weight; RSBI, rapid shallow breathing index; IWI modified, modified integrative weaning index.

Legend
The accuracy of each ventilatory variable in predicting weaning failure is presented as the area under the ROC curve with 95% confidence intervals.There were 35 out of 110 patients (32%) who failed prolonged weaning and remained ventilator dependent.
Abbreviations: SBT, spontaneous breathing trial; P/F ratio, the ratio of partial pressure of oxygen to fraction of inspired oxygen; LTCdyn, dynamic lung-thorax compliance; LTCdyn-MP, mechanical power normalized to dynamic lung-thorax compliance; VT/PBW, tidal volume normalized to the predicted body weight; RSBI, rapid shallow breathing index; IWI modified, modified integrative weaning index.

Table S1 :
Results of prolonged weaning -Comparison of patients with weaning failure and success b Legend Continuous variables are presented as median (-interquartile range [IQR]); categorical variables are presented as numbers (%).

Table S2 :
Start of weaning (first SBT) -Ventilatory and spontaneous breathing variables and indexes SBT, spontaneous breathing trial; PEEP, positive end-expiratory pressure; P/F ratio, the ratio of partial pressure of oxygen to fraction of inspired oxygen; VT/PBW, tidal volume normalized to the predicted body weight; LTCdyn, dynamic lung-thorax compliance; LTCdyn-MP, mechanical power normalized to dynamic lung-thorax compliance; CPAP, continuous positive airway pressure; RSBI, rapid-shallow breathing index; IWI modified, modified integrative weaning index.
c Legend Continuous variables are presented as median (-interquartile range [IQR]).a: P value for differences between the weaning failure and success group b: Student's t-test c: Mann-Whitney U-test Abbreviations:

Table S3 :
End of weaning (last SBT) -Ventilatory and spontaneous breathing variables and indexes cLegend Continuous variables are presented as median (-interquartile range [IQR]).a: P value for differences between the weaning failure and success group b: Student's t-test c: Mann-Whitney U-test Abbreviations: SBT, spontaneous breathing trial; PEEP, positive end-expiratory pressure; P/F ratio, the ratio of partial pressure of oxygen to fraction of inspired oxygen; VT/PBW, tidal volume normalized to the predicted body weight; LTCdyn, dynamic lung-thorax compliance; LTCdyn-MP, mechanical power normalized to dynamic lung-thorax compliance; CPAP, continuous positive airway pressure; RSBI, rapid-shallow breathing index; IWI modified, modified integrative weaning index.

Table S4 :
Cross-validated performance of ventilatory and spontaneous breathing indexes at the end of weaning (last SBT) analyzed to predict prolonged weaning failure Comparison of AUROC for MP density and LTCdyn with spontaneous breathing indexes at the end of weaning (last SBT) LegendResults of 2-times repeated, 5-fold cross-validation.Mean metrics of diagnostic accuracy (with 95% confidence intervals) based on threshold values associated with the Youden index (presented as the mean of the thresholds derived from the training sets).Abbreviations: PPV, positive predictive value; NPV, negative predictive value; PLR, positive likelihood ratio; NLR, negative likelihood ratio; DOR, diagnostic odds ratio; F1, F1 score; MCC, Matthews` correlation coefficient; VR, ventilatory ratio; LTCdyn, dynamic lung-thorax compliance; LTCdyn-MP, mechanical power normalized to dynamic lung-thorax compliance; VT/PBW, tidal volume normalized to the predicted body weight; RSBI, rapid shallow breathing index, IWI modified, modified integrative weaning index.

of the MP density and Comparison of LTCdyn and 1. VT/PBW 1. VT/PBW
Legend: Comparison of AUROC for MP density (expressed as Power indexrs 2.0 ) (Panel A) and LTCdyn (Panel B) with spontaneous breathing indexes at the end of weaning (last SBT) Abbreviations: AUROC, area under the receiver operating characteristic curve; MP, mechanical power; LTCdyn, dynamic lung-thorax compliance; VT/PBW, tidal volume normalized to the predicted body weight; RSBI, rapid shallow breathing index; IWI modified, modified Integrative weaning index, 95%CI, 95% confidence interval.

Table S5 :
Sensitivity analysis: Area under the ROC curve for each index analyzed to predict weaning failure at the start and end of weaning using a different PaCO2 threshold for ventilatory failure (> 50 mmHg)

Table S6 :
Sensitivity analysis: Area under the ROC curve for each index analyzed to predict weaning failure at the start and end of weaning in patients without COVID-19 pneumonia (N = 110)

Table S7 :
Correlations of ventilatory and spontaneous breathing indexes at last SBT with median spontaneous PaCO2 at the end of weaning Abbreviations: SBT, spontaneous breathing trial; LTCdyn, dynamic lung-thorax compliance; LTCdyn-MP, mechanical power normalized to dynamic lung-thorax compliance; VT/PBW, tidal volume normalized to the predicted body weight; RSBI, rapid shallow breathing index; IWI modified, modified Integrative weaning index.

Table S8 :
Sensitivity analysis: Correlations of ventilatory and spontaneous breathing indexes at last SBT with median spontaneous PaCO2 at the end of weaning in weaning success patients (N = 99) Abbreviations: SBT, spontaneous breathing trial; LTCdyn, dynamic lung-thorax compliance; LTCdyn-MP, mechanical power normalized to dynamic lung-thorax compliance; VT/PBW, tidal volume normalized to the predicted body weight; RSBI, rapid shallow breathing index; IWI modified, modified Integrative weaning index.

Table S9 :
Subgroup analysis -Comparison of patients with and without COPDLegendContinuous variables are presented as median (-interquartile range [IQR]); categorical variables are presented as numbers (%).§ : The total duration of mechanical ventilation, the period from intubation in the referring ICU until the completion of weaning at the weaning center.a: P value for differences between patients with and without COPD b: Mann-Whitney U-test c: Chi-squared test Abbreviations: COPD, chronic obstructive pulmonary disease; APACHE-II, Acute Physiology and Chronic Health Evaluation II score; SBT, spontaneous breathing trial; P/F ratio, the ratio of partial pressure of oxygen to fraction of inspired oxygen; VT/PBW, tidal volume normalized to the predicted body weight; LTCdyn, dynamic lung-thorax compliance; LTCdyn-MP, mechanical power normalized to dynamic lung-thorax compliance; RSBI, rapid-shallow breathing index; IWI modified, modified integrative weaning index; LOS, length of stay.